Mass spectrometry is widely used in many applications ranging from process monitoring to life sciences. Over the course of the last 60 years, a wide variety of instruments have been developed. The focus of new developments has been two fold: (1) a push for ever higher mass range with high mass resolution and MS/MS capability, and (2) on developing small, desktop MS instruments.
Mass spectrometers are often coupled with gas chromatographs (GC/MS) for analysis of complex mixtures. This is especially the case for volatile compound (VOC) and semi-volatile compound (semi-VOC) analysis. A GC/MS instrument typically has a gas inlet system (the GC would be part of this), an electron impact based ionizer [EI] with ion extractor, some optic elements to focus the ion beam, ion separation, and ion detection. Ionization can also be carried out via chemical ionization.
Ion separation can be performed in the time or spatial domain. An example for mass separation in the time domain is a time of flight mass spectrometer. Time domain separation is seen in commonly used quadrupole mass spectrometers. Here the “quadrupole filter” allows only one mass/charge ratio to be transmitted from the ionizer to the detector. A full mass spectrum is recorded by scanning the mass range through the “mass filter”. Other time domain separation is based on magnetic fields where either the ion energy or the magnetic field strength is varied, again the mass filter allowing only one mass/charge ratio to be transmitted and a spectrum can be recorded through scanning through the mass range.
An alternative concept is a mass spectrograph in which the ions are spatially separated in a magnetic field and detected with a position sensitive detector. The concept of a double focusing mass spectrograph was first introduced by Mattauch and Herzog (MH) in 1940 (J. Mattauch, Ergebnisse der exakten Naturwissenschaften, vol 19, page 170–236, 1940, which is incorporated herein by reference in its entirety).
Double focusing refers to the instrument's ability to refocus both the energy spread as well as the spatial beam spread. Modern developments in magnet and micro machining technologies allow dramatic reductions in the size of these instruments. The length of the focal plane in a mass spectrometer capable of VOC and semi-VOC analysis is reduced to a few centimeters.
The typical specifications of a small confocal plane layout Mattauch-Herzog instrument are summarized below:                Electron impact ionization, Rhenium filament        DC-voltages and permanent magnet        Ion Energy: 0.5–2.5 kV DC        Mass Range: 2–200 D        Faraday cup detector array or strip charge detector or electro optical ion detector        Integrating operational amplifier with up to 1011 gain        Duty Cycle: >99%        Read-Out time: 0.03 sec to 10 sec        Sensitivity: approximately 10 ppm with strip charge detector        
In traditional instruments the ion optic elements are mounted in the vacuum chamber floor or on chamber walls. The optics can also be an integral part of the vacuum housing lay-out. In small instruments, however, the ion optics can easily be built on a base plate which acts as an “optical bench”. This bench holds all components of the ion optics. The base plate is mounted against a vacuum flange to provide the vacuum seal needed to operate the mass spectrometer under vacuum. The base plate can also be the vacuum flange itself.
The ion detector in a Mattauch-Herzog layout is a position sensitive detector. Numerous concepts have been developed over the last decades. Recent developments focus on, solid state based direct ion detection as an alternative to previously used electro optical ion detection (EOID).
The electro optical ion detector (EOID) converts the ions in a multi-channel-plate (MCP) into electrons, amplifies the electrons (in the same MCP), and illuminates a phosphorus film with the electrons (emitted from the MCP). The image formed on phosphorus film is recorded with a photo diode array via a fiber optic coupler (see U.S. Pat. No. 5,801,380, which is incorporated herein by reference in its entirety). The electro-optic ion detector (EOID), is intended for the simultaneous measurement of ions spatially separated along the focal plane of the mass spectrometer. This device may operate by converting ions to electrons and then to photons. The photons form images of the ion-induced signals. The ions generate electrons by impinging on a microchannel electron multiplier array. The electrons are accelerated to a phosphor-coated fiber-optic plate that generates photon images. These images are detected using a photodetector array. The electro-optic ion detector (EOID), although highly advantageous in many ways, is relatively complicated since it requires multiple conversions. In addition, there may be complications from the necessary use of phosphors, in that they may limit the dynamic range of the detector. A microchannel device may also be complicated, since it may require high-voltage, for example 1 KV, to be applied. This may also require certain of the structures such as a microchannel device, to be placed in a vacuum environment such as 106 Torr. At these higher pressures of operation, the microchannel device may experience ion feedback and electric discharge. Fringe magnetic fields may affect the electron trajectory. Isotropic phosphorescence emission may also affect the resolution. The resolution of the mass analyzer may be therefore compromised due to these and other effects.
According to a different configuration, a direct charge measurement can be based on a micro-machined Faraday cup detector array. Here, an array of individually addressable Faraday cups monitors the ion beam. The charge collected in individual elements of the array is handed over to an amplifier via a multiplexer unit. This layout reduces the number of amplifiers and feedthroughs needed. This concept is described in detail in recent publications, such as “A. A. Scheidemann, R. B. Darling, F. J. Schumacher, and A. Isakarov, Tech. Digest of the 14th Int. Forum on Process Analytical Chem. (IFPAC-2000), Lake Las Vegas, Nev., Jan. 23–26, 2000, abstract I-067”; “R. B. Darling, A. A. Scheidemann, K. N. Bhat, and T.-C. Chen, Proc. of the 14th IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS-2001), Interlaken, Switzerland, Jan. 21–25, 2001, pp. 90–93”; and Non-Provisional patent application Ser. No. 09/744,360 titled “Charged Particle Beam Detection System”; all three of which are incorporated herein by reference in their entirety.
Other important references regarding spectrometers are “Nier, D. J. Schlutter Rev. Sci. Instrum. 56(2), page 214–219, 1985; and T. W. Burgoyne et. al. J. Am. Soc. Mass Spectrum 8, page 307–318, 1997; both of which are incorporated herein by reference in their entirety.
Alternatively, especially for low energy ions, a flat metallic strip (referred to as a strip charge detector (SCD)) on a grounded and insulated background can be used to monitor the ion beam. Again the charge is handed over to an amplifier via a multiplexer.
A very important ion detector array is disclosed in U.S. Pat. No. 6,576,899, which is also incorporated herein by reference in its entirety. It may be referred to as a shift register based direct ion detector.
That application defines a charge sensing system which may be used, for example, in a Mass Spectrometer system, e.g. a Gas chromatography—Mass spectrometry (GC/MS) system, with a modified system which allows direct measurement of ions in a mass spectrometer device, without conversion to electrons and photons (e.g., EOID) prior to measurement. In one case, it may use charge coupled device (CCD) technology. This CCD technology may include metal oxide semiconductors. The system may use direct detection and collection of the charged particles using the detector. The detected charged particles form the equivalent of an image charge that directly accumulates in a shift register associated with a part of the CCD. This signal charge can be clocked through the CCD in a conventional way, to a single output amplifier. Since the CCD uses only one charge-to-voltage conversion amplifier for the entire detector, signal gains and offset variation of individual elements in the detector array may be minimized.
In a Mattauch-Herzog layout the detector array, composed of either Faraday cup detector array or strip charge detector, or any other type of the aforementioned detectors, has to be placed at the exit of the magnet. This position is commonly referred to as the “focal plane”.
The Faraday cup detector array (FCDA) can be made by deep reactive ion etching (DRIE). The strip charge detector (SCD) can be made by vapor deposition. The dice with the active element (FCDA or SCD) is usually cut out of the wafer with conventional techniques such as laser cutting or sawing.
The FCDA or SCD dice needs to be held in front of the magnet and electronically connected to the multiplexer and amplifier unit called “Faraday Cup Detector Array”—“Input/Output”—“Printed Circuit Board” (FCDA-I/O-PCB) to read out the charge collected with the detector elements.
In traditional Mattauch-Herzog instruments the ion optics are placed on the vacuum chamber wall, and the position sensitive ion detector is mounted on the exit flange of the ion flight path. This arrangement is required as a result of having the magnet outside of the vacuum. The multiplexer and amplifier unit is also positioned outside of the vacuum chamber in the case of traditional Mattauch-Herzog instruments.
According to the present invention it is highly preferable that all parts of the ion optics are placed on the “base plate”, thus the position sensitive solid state based ion detector may be mounted against the same base plate. However, in certain embodiments of this invention, part of the magnetic section may be under vacuum, and part under atmospheric pressure. Further, the multiplexer and amplifier unit may also be positioned inside the vacuum chamber, which presents many advantages.
In general, the particles detected, usually ions, may be either negative or positive. A certain class of instruments has been used so far to detect positive particles, and a different class of instruments has been used to detect negative particles
According to the present invention a single instrument is being used to detect and measure both positive and negative particles.